A) Field of the Invention
The invention relates to a method to produce semiconductor light emitting devices.
B) Description of the Related Art
In the field of semiconductor light emitting devices, particularly those composed mainly of a Group-III nitride-based blue light emitting semiconductor that are represented as AlxInyGazN (0≦x≦1, 0≦y≦1, 0≦z≦1, x+y+z=1), there is recently a wider use of so-called vertical type semiconductor light emitting devices in which the growth substrate used for crystal growth of a semiconductor film has been removed and the electric current is caused to flow in the vertical direction.
A heterogeneous substrate such as a sapphire substrate that is low in price, high in lattice matching capability with AlxInyGazN, and useful for crystal growth of a high quality semiconductor film is commonly used as said growth substrate. However, sapphire is lower in refractive index for blue light than the semiconductor film. Accordingly, part of the blue light generated in the active layer in the semiconductor film is totally reflected back into the active layer, limiting the amount of light that is released out. A vertical type semiconductor light emitting device is free from a growth substrate to solve the problem with total reflection arising from the difference in refractive index and able to have a high light extraction efficiency.
In a vertical type semiconductor light emitting device, the support can be used as p-electrode, and therefore, it is not necessary to remove part of the semiconductor film that contains an active layer and to form n-electrodes and p-electrodes on the same face of a semiconductor film as in the case of a horizontal type semiconductor light emitting device. Thus, only n-electrodes exist on the light emitting side, making it possible to increase the light emitting area (active layer area) in a semiconductor light emitting device. Accordingly, the area shaded by the electrodes can be decreased.
A typical process to produce a vertical type semiconductor light emitting device is described below with reference to FIG. 11A to 11D. FIG. 11A shows a flow chart for the production of a vertical type semiconductor light emitting device. A growth substrate, such as a sapphire substrate, is prepared, and crystal of AlxInyGazN is grown to form a semiconductor film (step S101). MOCVD (metal organic chemical vapor deposition), for instance, is used to carry out the crystal growth for semiconductor film production. The semiconductor film formed should consist at least of a first semiconductor layer of a first electrical conduction type (n-type), an active layer that emits light, and a second semiconductor layer of a second electrical conduction type (p-type), arranged in this order from the growth substrate side.
Electron beam deposition, for instance, may be carried out to produce a metal layer on a semiconductor film surface (a metal multi-layered film to constitute p-electrodes and a joining layer), and the semiconductor film and the support are joined together with the metal layer sandwiched between them (step S102). The support is an electrically conductive substrate in the form of, for instance, a silicon wafer containing impurities or a plate made of or plated with a metal such as Cu and CuW or their alloy. This joining is commonly carried out by thermocompression bonding.
The growth substrate, such as sapphire substrate, is peeled and removed by the laser lift-off technique (step S103). A laser beam is applied from the growth substrate side so that some portions of the semiconductor layer of, for instance, GaN existing near the interface between the growth substrate and the semiconductor film are decomposed into Ga (metal) and N2 (gas). For instance, the entire face of the wafer is scanned by a linearly shaped laser beam to peel the growth substrate (see, for instance, Japanese Unexamined Patent Publication (Kokai) No. 2003-168820 as Patent document 1, and Japanese Unexamined Patent Publication (Kokai) No. 2006-073619 as Patent document 2).
In their previous application (Application Filing No. 2009-252902 as Patent document 3), the present inventors proposed a growth substrate peeling method in which the wafer is divided into regions, each planned to contain a semiconductor light emitting device, and laser beam pulses of a rectangular shape slightly larger than the regions are applied to each region.
FIG. 11B and FIG. 11C show a plan view and a cross section of a pulsed laser beam irradiation region in the growth substrate removal step (step S103). It contains a semiconductor film 20, a metal layer (p-electrodes and joining layer) 30, and a support 40 produced in this order on a growth substrate 10 in the semiconductor film production step (step S101) and the support production step (step S102). A pulsed laser beam, such as KrF excimer laser with a wavelength of 248 nm, is applied from the growth substrate side. The energy density may be, for instance, 920 mJ/cm2 at the beam-irradiated surface.
As shown in FIG. 11B, a rectangle shaped laser pulse beam is applied to scan them, for instance, in their short side direction (vertical direction in the figure) or in their long side direction (horizontal direction in the figure). When scanning the wafer, beam irradiated regions should overlap each other at their edges to prevent unirradiated regions from being left on wafer.
See FIG. 11C. The laser pulse used to irradiate wafer has a long side (laser beam width Q1) of, for instance, 540 μm and an overlap width Q2 of, for instance, 10 μm.
Subsequently, a process for removal of the growth substrate is carried out (step S104). FIG. 11D schematically shows a cross section of the laminated structure, which contains the semiconductor film 20, immediately after laser beam irradiation. The growth substrate 10 and the semiconductor film 20 are still joined together, sandwiching a metal Ga layer 50 separated out as a result of the decomposition of GaN caused by laser beam irradiation and the altered layer 60 which is part of the semiconductor film 20 altered by laser beam irradiation. The growth substrate 10 is removed by immersing the wafer in warm water of above 30° C., which is the melting point of Ga. The wafer may be immersed in HCl to dissolve Ga.
The semiconductor film 20 is removed in those portions existing between semiconductor light emitting devices (step S105) to produce grooves (streets) that isolate individual semiconductor light emitting device regions. Said removal of the semiconductor film 20 may be carried out by wet etching with an alkali solution such as TMAH (tetramethylammonium hydroxide) and KOH or dry etching such as RIE (reactive ion etching).
Then, n-electrodes are produced on the surface of the semiconductor film 20 (first semiconductor layer) exposed by removing the growth substrate 10 (step S106). In the Patent document 2, for instance, Au and Ti layers are laminated to produce n-electrodes.
The devices are separated (step S107). The metal layer 30 and the support 40 are cut along the streets produced in the semiconductor film 20 removal step (step S105) to separate individual semiconductor light emitting devices. Such methods as scribing/breaking, laser scribing, and dicing are used for the cutting operation.
The vertical type semiconductor light emitting device production method described above requires a substrate removal step (step S104) to be carried out after laser beam irradiation. Here, if said warm water or HCl cannot enter gaps, the growth substrate 10 will not be removed by the immersion in warm water or HCl in this step. If the altered layer 60 exists, furthermore, surface treatment such as mechanical polishing and dry etching with RIE will be required after removing the growth substrate 10.
The Ga layer 50 and the altered layer 60 that are produced by laser beam irradiation will not react with the etchant used for the semiconductor film 20 remove step (step S105), or they may show an etching rate different from that of the semiconductor film 20. In addition, the thickness of the metal Ga layer 50 (amount of the metal Ga separated out) and that of the altered layer 60 tend to be influenced by the warp of the wafer and the heat dissipation form the equipment, and they will be less uniform in the wafer as the energy of laser beam irradiation varies. If the semiconductor film 20 removal step (step S105) is carried out under such conditions, the degree of etching of the semiconductor film 20 will become irregular along the streets. As a result, residue of the semiconductor film remaining on the streets may cause inferior scribing or attach to the side face of the semiconductor light emitting device production regions to cause leak in later steps, resulting in a decrease in the yield. FIG. 12A gives a photograph that shows such irregular etching resulting from differences in surface conditions after the peeling of the growth substrate 10. Q3 and Q4 denote portions where the etching speed is higher and lower, respectively.
In some tests where the growth substrate 10 removal step (step S103) was carried out by the laser lift-off technique, it was found difficult to peel off peripheral portions of the wafer and remove the growth substrate 10 by laser beam irradiation. As the energy of laser beam irradiation for its removal was increased, the semiconductor film 20 was destroyed sometimes, and cracks took place in some semiconductor light emitting device production regions. In other cases, the semiconductor film 20 suffered cracks starting from portions between semiconductor light emitting device production regions (center of streets) where laser beam irradiation overlaps (multiple irradiation). FIG. 12B gives a photograph showing cracks that took place in multiple laser irradiated regions. In the photograph, cracks are surrounded by broken lines.